Refactoring of https://github.com/ZeusWPI/Gandalf/blob/2913ae7fcc1b8d690007a63c3ffec99c7c686188/app/controllers/registrations_controller.rb#L132

csv_importer.rb

# services/csv_importer.rb
class CSVImporter
  attr_reader :failed_rows, :success_count

  def initialize(content, separator, paid_column)
    @content = content
    @separator = separator
    @paid_column = paid_column

    @failed_rows = []
    @success_count = 0
  end

  def call
    CSV.parse(@content, col_sep: @separator, headers: :first_row) do |row|
      if parse_row(row)
        @success_count += 1
      else
        @failed_rows << row
      end
    end
  end

private

  def parse_row(row)
    registration = Registration.find_payment_code_from_csv(row.to_s)

    if registration.blank?
      return false
    end

    amount = amount_from_row(row)

    if amount.nil?
      return false
    end

    update_registration(registration, amount)

    if registration.is_paid
      RegistrationMailer.ticket(registration).deliver
    else
      RegistrationMailer.confirm_registration(registration).deliver
    end

    registration
  end

  def amount_from_row(row)
    Float(row[@paid_column].sub(',', '.'))
  rescue
    nil
  end

  def update_registration(registration, amount)
    registration.paid += amount
    registration.payment_code = Registration.create_payment_code
    registration.save!
  end

end

registrations_controller.rb

class RegistrationsController < ApplicationController
  
  # ...
  
  def upload
    @event = Event.find(params[:event_id])
    authorize! :update, @event

    separator   = params[:separator]
    csv_content = params[:csv_file].read.upcase
    paid_column = params[:amount_column].upcase

    importer = CSVImporter.new(csv_content, separator, paid_column)
    importer.call

    success_msg = "Updated #{'payment'.pluralize(importer.success_count)} successfully."

    if importer.failed_rows.any?
      flash.now[:success] = success_msg
      flash.now[:error] = "The rows listed below contained an invalid code, please fix them by hand."

      @csvheaders = importer.failed_rows.first.headers
      @csvfails = importer.failed_rows

      render 'upload'
    else
      redirect_to action: :index, success: success_msg
    end

  rescue CSV::MalformedCSVError
    redirect_to action: :index, error:  "The file could not be parsed. Make sure that you uploaded the correct file and that the column seperator settings have been set to the correct seperator."
  end

end